1. Field of the Invention
This invention relates to waterflooding procedures used for increasing the amount of oil which can be obtained from oil bearing subterranean rock formations containing water sensitive clays (argillaceous rock formations).
Water sensitive (hydratable) clays are found in many rock formations, e.g. sandstones and carbonate formations, and are generally of the smectite group, e.g. montmorillonite, beidelonite, nontronite, saponite, hectorite and sauconite.
2. Discussion of the Prior Art
Enhanced hydrocarbon (oil) recovery techniques (sometimes called secondary or tertiary recovery techniques) are used to recover additional hydrocarbons from subterranean rock strata once the hydrocarbons no longer flow from the strata under their own pressure in commercially significant quantities. The signal most commonly used method of enhanced recovery is water flooding. In this process water is injected into the formation through one or more injection or input wells in order to displace hydrocarbons from the reservoir and the displaced hydrocarbons are recovered from one or more production wells drilled into the same formation at suitable distances from the injection wells. A good discussion of the principles of such waterflooding practices can be found in a monograph of the Society of Petroleum Engineers section of the American Institute of Mining Engineering entitled "The Reservoir Engineering Aspects of Waterflooding" by F. F. Craig, and a publication entitled "Enhanced Recovery of Residual and Heavy Oils", Second Edition (1980) Edited by M. M. Schumacher and published by Noyes Data Corporation, the disclosures of which are incorporated herein by reference.
The water required for such techniques is usually taken from a readily available natural source such as a river, lake or subterranean formation. In the course of waterflooding it is common practice to treat the waters (which are normally brines) used for injection purposes to prevent plugging of the formation caused by such factors as mineral scales, suspended mineral solids, or bacteria. Normal techniques include chemical or physical treatments to remove certain dissolved cations or anions which may be incompatible with waters native to the reservoir being waterflooded. An example of incompatible dissolved chemical species would be the injection of waters having high sulfate or carbonate concentrations into a reservoir containing in-situ waters rich in calcium, strontium or barium. The potential for forming low solubility salts of calcium carbonate, calcium sulfate, strontium sulfate or barium sulfate under the circumstances must be avoided by chemical treatments to prevent scale disposition and the resulting loss in injectivity that occurs when in-situ scales are formed in the reservoir.
Another frequently used technique involves filtration of suspended matter that may be present in the injection waters. Also, organic materials such as trace hydrocarbons may be present in the injection waters if these waters are obtained from other subterranean aquifers. These hydrocarbons are normally removed by mechanical equipment to prevent their disposition in the reservoir rock thereby preventing alterations of reservoir rock wettability and the ensuing reduction in the relative permeabilities of the reservoir rock to waters.
Another form of treatment of injection waters involves the addition of bactericides to prevent plugging caused by bacterial growth in the injection wellbores or hydrocarbon reservoir. Also removal of dissolved gases such as oxygen (that may have been introduced into the injection waters as a result of other treatment procedures) is frequently practiced to prevent potential corrosion by-products, or oxidation of more soluble chemical species to lower soluble species (i.e. sulfides to sulfates).
In all of the above cases the primary objective is to render the injection waters compatible with in-situ reservoir fluids and prevent damage to the reservoir. As such these methods are not designed to enhance the recovery of hydrocarbons exept in that they ensure optimal rates of water injection by preserving permeability of the reservoir being waterflooded.
To improve further the recovery of hydrocarbons, a number of chemically modified waterflooding procedures have been developed. In the so-called alkaline waterflooding practices, a caustic solution such as sodium or potassium hydroxides or silicates are admixed with the injection waters so that these chemicals combine with fatty acid or hydroxyl-containing species of the reservoir hydrocarbons and are converted to crude detergents or surfactants. These in turn reduce the interfacial forces between the reservoir rock and fluids to promote improved recovery of the hydrocarbons from the reservoir. Unfortunately many reservoirs contain appreciable quantities of native clays which swell when contacted by high pH fluids so that the benefits of interfacial tension lowering using caustic solutions are negated by a loss in rock permeability caused by clay swelling.
Another procedure for augmenting waterflood practices to improve hydrocarbon recovery involves the use of polymers to viscosify (thicken) the injection waters. This method of treatment decreases the disparity in mobility between the displacing phase (injected fluid) and the displaced phase (hydrocarbons) thereby reducing the tendency of water to bypass the hydrocarbons located in the less permeable sectors of the reservoir. Again, the costs associated with adsorption and shear degradation of the polymer within the reservoir, injection problems caused by the highly-viscous fluids, and the physiochemical limitations of many commercially available polymers frequently render the overall process uneconomic.
An important factor in controlling the amount of hydrocarbons (oil) recovered by displacement processes such as normal waterfloods or chemically-modified waterfloods is described by the mobility ratio of the displacing (injected) fluids and the displaced (produced) oil. Current convention defines the mobility ratio (M) as follows: ##EQU1## wherein: Kw represents the permeability of the formation to the displacing (injected) phase within that portion of the reservoir wetted by this phase;
Knw represents the permeability of the oil saturated portion or the formation to the displaced oil; PA1 .mu.nw represents the viscosity of the displaced oil (non-wetting phase); and PA1 .mu.w represents the viscosity of the displacing (injected) phase. PA1 K.sub.r is the relative permeability PA1 Ke is the effective permeability PA1 Ka is the absolute permeability
It is well known that a reduction of the mobility ratio favours increased hydrocarbon recovery by reducing the amount of fluids which must be injected to recover the oil. By conventional use, mobility ratios of less than unity are termed "favourable," and those greater than unity are "unfavourable". From the above equation it can be shown that either lowering the mobility of the displacing phase or raising the mobility of the displaced phase affords the opportunity to lower the mobility ratio. This expression supports the rationale behind many of the current enhanced oil recovery schemes. For example, in the previously described polymer augmented waterflood process the injected fluids are made more viscous than the displaced fluids. This would have the effect of lowering the mobility ratio and increasing the amount of hydrocarbons recovered. Similarly, in miscible processes such as gas injection schemes the effective viscosity of the hydrocarbons are lowered when the injected gas dissolves in the hydrocarbon phase.
For a more detailed explanation of the many methods and mechanisms of enhanced oil (hydrocarbon) recovery reference is made to "Enhanced Oil Recovery, 1976 & 1984 Editions, published by the National Petroleum Council, Washington, D.C.
While it is clear from the above that numerous attempts have been made to improve waterflood techniques, an entirely satisfactory treatment method has not yet been developed, particularly when the reservoir to be treated contains hydratable clays which are sensitive to differences between connate water and treatment solutions.